1. Field of the Invention
In the industrialized world, production equipment is becoming more and more complicated and expensive. Customers have become, increasingly, more demanding of high quality cemented carbide necessary for competitive production. The life of a piece of equipment mostly depends upon the durability of a few vital components exposed to wear, fatigue or corrosion. This is an important reason for concentrating on the improvement of these few expensive components.
Cemented carbide has greater wear resistance and strength than tool steels. This is why there are many tungsten carbide applications in industry today. In spite of the five to ten times additional cost, the performance justifies the additional expense. Still these carbides can be very expensive to replace. This invention relates to the process of resintering cemented carbide products to make them more durable and to prolong their life. Specifically, it is related to sintered carbide products that are loaded and unloaded cyclically and ultimately crack in a type of fatigue failure.
2. Prior Art
The process of sintering carbide has been known in the industry for over 30 years. Sintering consists of bonding powdered carbide with a binder metal from the iron group (iron, nickel, or cobalt) under high temperatures (around 1400 degrees Centigrade), in a controlled atmosphere, and with a controlled heat cycle. This forms a product with extremely high hardness and compressive strength. The strength and hardness can be varied with different amounts and types of binder metal, additives and sizes of the powdered carbide grains. Small amounts of Tantalum and Titanium are sometimes added to change the properties of the final product. After sintering, the cemented carbide tooling has a rough, and slightly irregular, surface which needs be ground to finished dimensions.
Cemented carbide has, among other things, high strength at elevated temperatures and is very resistant to wear. This makes sintered carbide valuable in cutting tool applications, metal forming tooling, and in the high pressure, high temperature industry.
It has been a major objective of the high pressure, high temperature industry to increase the life of their cemented carbide tooling. In that industry, cemented carbide tooling represents a significant expense of operation. The problem is that the tooling fails under the necessary operating conditions after a limited number of cycles. The carbide fails due to internal cracks which begin with the first cycle and are propagated exponentially with each successive cycle. Complete failure occurs when these cracks, reach the surface of the carbide and the tooling can no longer withstand the operational loads. Upon fracture, the carbide tool is no longer of use and must be replaced. The scrap value is a small portion of the original cost.
Cemented carbide users have tried to prolong the life of their tooling by various post sintering processes with mixed results. One of these processes is grinding or lapping the surface of the part, after some use, to remove some surface defects and adverse chemistry. Others have tried heat treating cemented carbide parts short of resintering temperatures with some success. But historically, the resintering of cemented carbide has been unexplored and considered an unlikely area for improving product. The conventional wisdom was that nothing could be done to heal internal cracks once they had started.
It is, therefore, the object of this invention to provide a method of improving the life of such cemented carbide parts through a resintering process and thereby reducing the cost of operating with cemented carbide.